METHOD AND APPARATUS FOR CHIEF RAY ANGLE CORRECTION USING A DIFFRACTIVE LENS
Methods and apparatus reduce the chief ray angle incident on a pixel array of an imaging device by the use of a diffractive lens.
Embodiments of the invention relate to correcting the angle of refraction of light.
BACKGROUNDSolid state imaging devices, e.g., CCD, CMOS, and others, include a lens or series of lenses to direct incoming light onto a focal plane array of pixels. Each one of the pixels includes a photosensor, for example, a photogate, photoconductor, or photodiode, overlying a substrate for accumulating photo-generated charge in an underlying portion of the substrate. The charge generated by the pixels in the pixel array is then read out and processed to form an image.
A transparent material 130 having an index of refraction n
Light rays 140a, 140b, 140c are generally focused by the focusing lens 110 into a conical bundle of light rays 140. The light ray in the center of the bundle of light rays 140 is known as the chief ray 140a and the angle of the chief ray is known as the chief ray angle. The chief ray angle is measured in relation to the normal of the planar surface 156 of the focusing lens 110, with an angle of zero degrees being perpendicular to the planar surface 156. As shown in
However, as shown in
What is needed is a system and method by which light rays having a high chief ray angle are redirected from a focusing lens onto a pixel array 120 of an imaging device.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration of specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them, and it is to be understood that structural, logical or procedural changes may be made to the specific embodiments disclosed herein.
To redirect the bundle of light rays 140, the period p, i.e., the width, of the grooves 350 located closer to the center 354 of the diffractive lens 300 is wider than the period p of the grooves 350 located farther away from the center 354 of the diffractive lens 300. In the embodiment shown in
In the embodiment shown in
Therefore, because the period p of the grooves 350 changes according to the distance of a groove 350 from the center 354 of the diffractive lens 300, the angle of the second side 350b, known as the blaze angle BA, also changes according to the distance from the center 354 of the diffractive lens 300. The increase in the blaze angle BA and the decrease in the groove 350 period p at grooves further from the center 354 causes light rays striking the diffractive lens 300 at a location further from the center 354 to be diffracted to a greater degree than light rays striking the diffractive lens 300 at a location closer to the center 354.
Although the diffractive lens 300 and focusing lens 110 are shown in
The transparent material 130 arranged between the focusing lens 110 and the pixel array 120 has an index of refraction n
The diffractive lens 300 and the focusing lens 110 may be made of the same materials, e.g., glass or polymer. Alternatively, the indexes of refraction n
As shown in
In one embodiment, a minimum of about four grooves 350 over the bundle may be used to diffract light rays 140 exiting the radially outer part of the diffractive lens 300 towards pixel array 120. There is no visible transition in the image produced by the pixel array 120 due to the grooves 350. The diffractive lens 300 can thus decrease or keep constant the chief ray angle of light exiting the diffractive lens 300.
The period p of the grooves 350 may vary depending on the amount of diffraction that is required for incoming light. In one embodiment, the period p of the grooves 350 may be between about 0.4 to about 4.0 μm, although the periods p will vary radially within a single lens 300 as described above. The depth d of the grooves 350 follows the required period and blaze angle for given diffraction/deflection angle.
In another embodiment, the period p used to refract a light ray at particular portion of the diffractive lens 300 may be determined by equation (1):
p=mλ/(n
where m is the diffraction order, λ is wavelength of the light ray, p is the period of the groove 350, n
For a specific case where it is desired that Θ
p=λ/(sin Θ(n
Furthermore, if the diffractive lens 300 is made of glass and the transparent material 130 is made of air, and n
p≈2λ/(sin Θ) (3)
where Θ is the angle of light both before and after passing through the diffractive lens 300/transparent material 130 interface. Θ can then be easily related to the desired angle of light striking the pixel array at any specific portion of the pixel array 120 and the period p and blaze angle BA of the grooves 350 can be adjusted accordingly and gradually at various radii of the diffractive lens 300.
For example, if the maximum desired angle of light striking the pixel array 120 is 35 degrees (Θmax=35 degrees), then for λ=0.55 um, the period p of the smallest groove 350, located at the edge of the diffractive lens 300, would be 1.9 μm. The diffractive dispersion of light in this example can be calculated for the visible spectrum from 0.42 μm to 0.65 μm wavelength to Θ0.42=26 deg, Θ0.55=35 deg, Θ0.65=43 degrees. Thus, the dispersion of light having the maximum angle of 35 degrees is about ±8.5 deg for the visible spectrum.
In the embodiment shown in
The decrease in the groove 650 period p at grooves further from the center 654 causes light rays striking the diffractive lens 600 at a location further from the center 654 to be more diffracted at a greater angle than light rays striking the diffractive lens 600 at a location closer to the center 654.
The depth d of the grooves 650 may be configured so that the optical path difference between rays passing through the grooves and passing through the bumps in perpendicular transmission is an integer multiple of the center wavelength of the imaging device to cause constructive interference. Constructive interference may be achieved where i is an integer value and where d=Iλ/(n
In the embodiment shown in
The grooves 350, 650, 1050 described herein may be formed by precision single point diamond turning, although the limited diamond radius may not allow for certain features, such as edge sharpness of the grooves 350, 650, 1050, or certain sizes to be achieved. In other embodiments, the grooves 350, 650, 1050 may be formed by laser or electron beam writing, gray scale lithography, or multilevel kinoforms using multiple binary marks and subsequent replication and/or etching steps using a photoresist and ultraviolet cured polymer and glass, respectively.
The diffractive lens 300 may be included in wafer level optical modules formed by aligning and assembling a wafer containing multiple lens structures to a wafer containing multiple imager dies. The wafer containing multiple lens structures may be spaced apart from the wafer containing multiple imager dies by a spacer wafer. The assembled wafers may then be cut to form individual imager modules. The diffractive lenses may be included as a separate wafer or may be a part of the wafer containing the multiple lens structures.
Signals from the imaging device 700 are typically read out a row at a time using a column parallel readout architecture. The timing and control circuit 732 selects a particular row of pixels in the pixel array 120 by controlling the operation of a row addressing circuit 734 and row drivers 740. Signals stored in the selected row of pixels are provided to a readout circuit 742. The signals are read from each of the columns of the array sequentially or in parallel using a column addressing circuit 744. The pixel signals, which include a pixel reset signal Vrst and image pixel signal Vsig, are provided as outputs of the readout circuit 742, and are typically subtracted in a differential amplifier 760 and the result digitized by an analog to digital converter 764 to provide a digital pixel signal. The digital pixel signals represent an image captured by pixel array 120 and are processed in an image processing circuit 768 to provide an output image.
System 800, e.g., a digital still or video camera system, generally comprises a central processing unit (CPU) 802, such as a control circuit or microprocessor for conducting camera functions, that communicates with one or more input/output (I/O) devices 806 over a bus 804. Imaging device 700 also communicates with the CPU 802 over the bus 804. The processor system 800 also includes random access memory (RAM) 810, and can include removable memory 815, such as flash memory, which also communicates with the CPU 802 over the bus 804. The imaging device 700 may be combined with the CPU processor with or without memory storage on a single integrated circuit or on a different chip than the CPU processor. In a camera system, a focusing lens 110 in conjunction with a diffractive lens according to various embodiments described herein may be used to focus image light onto the pixel array 120 of the imaging device 700 and an image is captured when a shutter release button 822 is pressed.
While embodiments have been described in detail in connection with the embodiments known at the time, it should be readily understood that the claimed invention is not limited to the disclosed embodiments. Rather, the embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described. For example, while some embodiments are described in connection with a CMOS pixel imaging device, they can be practiced with any other type of imaging device (e.g., CCD, etc.) employing a pixel array or a camera using film instead of a pixel array.
Claims
1. An imaging structure, comprising:
- a focusing lens structure for focusing an image, the focusing lens having a light exiting side;
- a pixel array for capturing the image focused by the focusing lens structure;
- a diffractive lens arranged between the focusing lens structure and the pixel array; and
- a transparent material arranged between the diffractive lens and the pixel array,
- wherein the diffractive lens comprises: a light entering first side facing towards the light exiting side of the focusing lens structure, and a second side facing towards the pixel array, the second side comprising a grating, the grating comprising a plurality of grooves arranged in concentric rings, wherein a period of the grooves decreases according to an increase in distance from a center of the grating.
2. The imaging device of claim 1, wherein the first side is planar.
3. The imaging device of claim 2, wherein a minimum period of each of the plurality of grooves is between about 0.4 to about 4.0 micrometers.
4. The imaging device of claim 2, wherein a period of at least one of the plurality of grooves is determined by the equation:
- p=mλ/(nDL sin ΘDL−nTM sin ΘTM)
- where m is a diffraction order, λ is wavelength of a light ray entering the diffractive lens, p is a period of the groove, nDL is an index of refraction of the diffractive lens, nTM is an index of refraction of the transparent material, ΘDL is an angle of the light ray in the diffractive lens with respect to the first side of the diffractive lens, and ΘTM is an angle of the light ray in the transparent material with respect to the second side of the diffractive lens.
5. The imaging device of claim 4, wherein ΘTM=ΘDL and m=1.
6. The imaging device of claim 5, wherein the diffractive lens comprises glass or polymer and the transparent material comprises air, and wherein nDL−nTM is approximately equal to 0.5.
7. The imaging device of claim 1, wherein all of the grooves have the same depth.
8. The imaging device of claim 1, wherein the grooves are substantially triangular in shape.
9. The imaging device of claim 8, wherein blaze angles of substantially triangular grooves located farther from a center of the grating are larger than blaze angles of substantially triangular grooves located closer to the center of the grating, wherein the blaze angles are measured with regard to the first side.
10. The imaging device of claim 8, wherein the substantially triangular grooves comprise a first side arranged substantially perpendicular to the first surface and a second side that slopes in a downward direction away from a center of the grating.
11. The imaging device of claim 1, wherein the grooves are substantially rectangular in shape.
12. The imaging device of claim 11, wherein the substantially rectangular grooves comprise a first side and a second side arranged substantially perpendicular to the first surface and a third side arranged substantially parallel to the first surface.
13. The imaging device of claim 1, wherein the grooves are substantially trapezoidal in shape.
14. The imaging device of claim 13, wherein the substantially trapezoidal grooves comprise a first side arranged substantially parallel to the first surface and a second side and a third side sloping downwards and inwards towards the first side.
15. The imaging device of claim 1, wherein the focusing lens and the diffraction lens are in direct contact with each other.
16. The imaging device of claim 1, wherein the diffraction lens is integral to the focusing lens.
17. The imaging device of claim 1, wherein an index of refraction of the diffractive lens is approximately equal to an index of refraction of the focusing lens and wherein an index of refraction of the transparent material is less than the index of refraction of the diffractive lens.
18. A camera system employing the imaging device of claim 1.
19. An imaging module, comprising:
- an imager die comprising a pixel array;
- a focusing lens structure for focusing an image onto the pixel array;
- a transparent material arranged between the focusing lens and the pixel array; and
- a diffractive grating comprising a plurality of grooves arranged between said focusing lens structure and said transparent material,
- said focusing lens structure, diffractive grating, and transparent material being joined as a modular structure.
20. The imaging module of claim 19, wherein grooves located closer to a center of the grating are wider than grooves located farther from the center of the grating and wherein all of the grooves have the same depth.
21. The imaging module of claim 19, wherein the grooves comprise a first side arranged substantially perpendicular to the pixel array and a second side that slopes in a downward direction away from a center of the surface.
22. The imaging module of claim 19, wherein the grooves comprise a first side and a second side arranged substantially perpendicular to the pixel array and a third side arranged substantially parallel to the pixel array.
23. The imaging module of claim 19, wherein a period of at least one of the plurality of grooves is determined by the equation:
- p=mλ/(nDL sin ΘDL−nTM sin ΘTM)
- where m is a diffraction order, λ is wavelength of a light ray within the focusing lens, p is a period of the groove, nDL is an index of refraction of the focusing lens, nTM is an index of refraction of the transparent material, ΘDL is an angle of the light ray in the focusing lens with respect to the pixel array, and ΘTM is an angle of the light ray in the transparent material with respect to the pixel array.
24. A method of forming an imager module, comprising:
- providing a first wafer containing a plurality of imager dies;
- providing a second wafer containing a plurality of lens structures for focusing an image onto the plurality of imager dies;
- providing a transparent material;
- providing a diffractive grating comprising a plurality of grooves;
- coupling the first wafer to the second wafer to form a structure in which the diffractive grating is arranged between the first wafer and the second wafer and the transparent material is arranged between the diffractive grating and the first wafer; and
- dividing the structure into a plurality of imager modules, each module comprising an imager die, a lens structure, a diffractive grating, and a transparent material.
25. The method of claim 24, further comprising forming the plurality of grooves by laser or electron beam writing.
26. The method of claim 24, further comprising forming the plurality of grooves by gray scale lithography and subsequent replication and/or etching steps.
27. The method of claim 24, further comprising forming the plurality of grooves by diamond turning and a subsequent lithography technique.
28. The method of claim 24, further comprising forming multilevel kinoform elements by multiple binary marks.
29. The method of claim 24, wherein grooves located closer to a center of the grating are wider than grooves located farther from the center of the grating.
30. The method of claim 24, wherein the grooves are substantially triangular in shape and wherein blaze angles of substantially triangular grooves located farther from a center of the grating are larger than blaze angles of substantially triangular grooves located closer to the center of the grating.
31. The method of claim 24, wherein the grooves are substantially rectangular or substantially trapezoidal in shape.
Type: Application
Filed: Jul 1, 2008
Publication Date: Jan 7, 2010
Inventors: Jacques Duparre (Jana), Jeff Viens (Boise, ID)
Application Number: 12/166,077
International Classification: G02B 5/18 (20060101); B29D 11/00 (20060101);